Add npm virtual repositories (#152)

npm clients need one registry for internal and public packages, but npm virtuals have no merger, so a local npm repo and npmjs cannot be served together.

- merge packuments across members: union versions (member order wins), first-defined dist-tags minus ones naming missing versions
- serve full and abbreviated (`install-v1`) packuments, scoped names included, via the shared virtual merge cache
- point tarballs at `<virtual>/<member>/<path>` and 302 them to the owning member
- reject an invalid Host with 400; serve virtual and local packuments `no-store` with `Vary: Host, X-Forwarded-Proto, Accept`
- return 502 when a member errors; skip members without the package
- add `docs/npm/virtual.md` and a stock `node:22` e2e-docker case

Reviewed-on: #152
Co-authored-by: unkin-agent <unkin-agent@unkin.net>
Co-committed-by: unkin-agent <unkin-agent@unkin.net>

internal/server/server.go

@@ -166,6 +166,7 @@ func (s *Server) routes() chi.Router {
 	r.Mount(tfregistry.MountPath, s.tfRegistry.Routes())
 
 	proxyHandler := v1.NewProxyHandler(s.engine, s.virtEngine, s.db, s.store, s.localHandler)
+	s.virtEngine.NPMLocal = proxyHandler.LocalNPMPackument
 	r.Mount("/api/v1", proxyHandler.Routes())
 	r.Mount("/v2", proxyHandler.DockerV2Routes())